1. Field of the Invention
The present invention relates to a color library system for a color designer engaged in metallic-paint-color design capable of easily visually detecting an approximate color by quickly retrieving a metallic paint color to be changed depending on an observing angle by a computer and displaying a retrieved result on a display device (monitor) of the computer as a computer graphic of the metallic paint color.
2. Background and Prior Art of the Invention
A metallic paint color is frequently used which is obtained by adding a colored pigment, a scaly brilliant pigment (such as aluminum flake or pearl flake), and micro titanium dioxide using Rayleigh scattering to a binder in order to improve the design effect in the fields of paint and printing. The maximum feature of the coloring is to make a person feel various textures such as metallic feeling and pearl feeling because brightness, chroma, and hue are changed depending on an observing angle. FIG. 1 shows the paint-film structure and the multi-angle gloss distribution of a general metallic color. By applying light from a direction of 45xc2x0, the light is directionally reflected by a brilliant pigment in a paint film. Therefore, there is a feature that the high-light side becomes bright and the shade side becomes dark. Moreover, an angle is generally shown by an open angle from specular reflected light (this is referred to as declination) by assuming the specular reflected light and 0xc2x0.
Because development of many new colors is requested from customers (automobile companies, general industrial companies, and construction companies) every year, pain makers prepare a plurality of paint colors matching images of the customers and continuously propose them. However, there is a problem that it is impossible to prepare a sufficient number of paint colors because it takes a lot of time to individually prepare new paint colors every time. Particularly in recent years, because improvement of the products power of automobiles is requested, the importance for the designing characteristic of an exterior color that is most remarkable by consumers is raised. For a designer of an automobile maker using the strategy for improving the products power of automobiles, a step of observing a plurality of colors approximate to image colors of the designer and selecting a better color has been important more and more.
Therefore, paint companies has respectively efficiently prepared a color desired by a customer by detecting a paint color having the color and texture matching the image desired by the customer out of a enormous number of stock colors prepared in the past and tuning the color and texture by using the paint color as a start color and using various color materials.
Moreover, the stock colors also have a role of efficiently handing down the experience, skill, and sense of a veteran color designer to a new color designer, which are properties of a paint company. Therefore, 500 or more new colors are registered every year.
However, when the number of stock colors exceeds 1,000, disadvantages occur that it takes a lot of time to find out a color matching the image of a customer only through visual observation, the work efficiency is deteriorated instead, and stock colors which are also the property of a paint company cannot be effectively used.
Therefore, Japanese Patent Application No. 167960/1987 discloses a method of storing a colorimetric value in a memory of a computer by using a colorimeter and retrieving an approximate color by using a color difference (JIS Z8730) as a parameter. However, though it is possible to achieve an object by computing the color difference between colorimetric values from one angle when using a solid color (color shown by only a pigment which excludes a flake brilliant pigment and whose color is not changed depending on an observing direction), it is impossible to achieve the object for a metallic color which is changed depending on an observing direction by the above method.
The present invention provides a system for quickly computing an approximate color of a metallic paint color which is changed depending on an observing angle and displaying the computed approximate color on a monitor as a computer graphic of the paint color, particularly displaying an approximate color of a metallic paint color preferred by a color designer.
The present invention mainly aims at a computer system to be used by a designer engaged in design of a paint of an exterior color for an automobile. However, when considering that metallic colors containing flake pigments are used for some railroad cars and building armors in recent years, the present invention can be effective means in fields of general industry and construction in future.
Main components of the present invention are described below. That is, the main components are composed of.
1. a procedure for storing stock colors of metallic colors as the digital information for colorimetric values, color classification codes, and computer graphic images of metallic paint colors by using a colorimeter;
2. an algorithm for quickly retrieving an approximate color of an optional metallic paint color in accordance with a prepared color classification code; and
3. an algorithm for computing an weighted color difference so as to match a visual approximate color in order to compute color differences from a high-light (bright color close to specular reflected light) of a metallic paint color up to a shade (dark color at the opposite side).
A specific treatment method is described below.
A method for measuring a metallic paint color generally uses a multi-angle colorimeter. Various types of colorimeters are marketed. Any colorimeter measures an object under an optical condition according to a general expression of xe2x88x9245xc2x0/xc3x971, xc3x972, xc3x973, xc3x974, xc3x975, . . . , and xc3x97n. In this case, xe2x88x9245xc2x0 denotes the angle of incident light. However, the angle may be shown as 45xc2x0 with no negative symbol. Symbol xi denotes a received angle. There are a colorimeter that continuously measures xi and a portable multi-angle colorimeter that discretely measures xi every 10xc2x0 to 50xc2x0. Moreover, there are a colorimeter for measuring XYZ in accordance with the photoelectric tristimulus colorimetry and a colorimeter for measuring a spectral reflectance. Because a continuous-measuring colorimeter can measure an object every optional angle, it can measure from a high-light up to a shade in detail. However, the colorimeter is large and expensive and requires a lot of time for measurement and a lot of measured data. Therefore, it is not suitable for the present system. To obtain the number of measurements necessary and sufficient for the present system, a portable multi-angle spectrophotometer is suitable which can measure from the high-light side up to the shade side at five angles or more. It is recommended to use the method disclosed in Japanese Patent Laid-Open No. 10045/1998 in order to obtain a regression formula for estimating wide-range colors from high-light to shade.
Moreover, to obtain a computer graphic of a metallic paint color from the obtained regression formula, it is possible to use an already-publicly-known method. That is, the above computer graphic can be obtained by converting a reflectance estimated by the regression formula into tristimulus values XYZ in accordance with the expression specified in JIS Z8701 and then, into RGB of a monitor. It is possible to convert XYZ into RGB by using a determinant in accordance with values of XYZ obtained by measuring the spectral brightness of each of R, G, B, and WHITE of the monitor by a spectral-radiant luminance meter. Moreover, to more accurately perform the conversion from XYZ into RGB, it is necessary to obtain the function of luminescent brightness (this is referred to as xcex3 function) to input voltages of R, G, and B of the monitor for correction. To convert XYZ into RGB, it is allowed to directly measure the characteristic of a monitor to be directly. Moreover, it is allowed to simply use a conversion formula announced by CIE (International Commission on Illumination) as an approximation to the NTSC color coordinate to be frequently used for a television by a monitor.
Then, the spectral reflectance of xn angle is converted into XYZ by using the expression in JIS Z8701 and moreover converted into Lab* by using the conversion formula in JIS Z8729.
Then, to quickly retrieve a metallic paint color to be described later, a color classification code is computed in order to prevent computation from requiring a lot of time because much unnecessary computation must be executed including a silver metallic color and a white pearl color completely different from a blue metallic color as objects for approximate-color retrieval when computing all approximate colors of stock colors when retrieving the blue metallic color. Because a metallic color has a characteristic that the color changes depending on an observing angle, color classification codes different in high-light and shade are necessary and thereby, approximate-color retrieval is complicated. Therefore, the present invention uses a typical color Lab* of metallic paint colors disclosed in Japanese Patent Laid-Open No.211569/1999.
To obtain a color classification code of a metallic color from the typical color Lab* of metallic colors, it is possible to use a chromatic method. A chromaticity value meeting the above purpose is converted from Lab* into Lch* in accordance with JIS Z8729 and then, it is possible to use metric chroma c* or metric hue h*. Moreover, it is allowed to convert the chromaticity value from Lab* which is a typical color of metallic paint colors into Hue and Tone values by using the M*MC made by Nippon Color and Design Research Institute Inc. used in Japanese Patent Laid-Open No.211569/1999 and then, use the color classification frequently used for exterior color design of automobiles. FIG. 2 shows an example. Color classification codes are decided by dividing all color areas into 5 to 100 (both included), preferably into 5 to 50 (both included) by any method.
Computer-graphic images of the above computed paint-color name, Lab* of n angle, color-classification code, and metallic paint color are stored in a memory of a computer.
Then, a specific method for retrieving an approximate color is described below. A metallic paint color whose approximate color will be retrieved is prepared. This is referred to as the metallic paint color. When the metallic paint color is included in stock colors, it is possible to call it by a paint color name. When the color is not included in the stock colors, the metallic paint color is temporarily stored in a memory as digital information. The typical color Lab* of the metallic paint color is computed and then, Hue-Tone values are computed to obtain a color classification code to which the color belongs.
It is possible to quickly retrieve an approximate color by extracting only colors having color classification codes same as those of metallic paint colors stored in a computer and computing the color difference between the colors.
A color-difference computation for obtaining an approximate color of a metallic paint color is described below in detail. The metallic paint color has a feature that it is bright in high-light and dark in shade. Though it is possible to compute an approximate color of a solid color shown by only a pigment including no flake pigment in accordance with the color difference formula in JIS Z8730, a case of a metallic paint color is insufficient.
As a result of studying various color difference formulas for retrieving an approximate color of a visually-suitable metallic paint color, the following laws are found.
(1) Though the high-light side has a value for a brightness L* to greatly exceed 98 of white and a value exceeding 200 is frequently obtained from a silver metallic color, the difference between brightnesses cannot be clearly detected through visual observation. For example, even if a color difference at the high-light side of a silver metallic color has a large value of 200 or more, the difference cannot be detected in visual color difference. To make the color difference at the high-light side coincide with visual color-difference feeling, it is preferable to multiply the difference dL* between brightnesses at the high-light side by a weighted factor between 0.3 and 1.0 (both included), more preferably a weighted factor between 0.5 and 0.8 (both included).
(2) Because a brilliant pigment such as aluminum flake or mica flake shines in the case of high-light, the colorimetric value of the high-light is generally large but the colorimetric value of the shade side is small because no flake pigment shines. In the case of visual observation, however, an approximate color is detected because of further laying stress on the color difference between shades. Thereby, when considering three divisions of high-light side, intermediate face, and shade, it is preferable that a color-difference weighted factor ranges between 0.5 and 1.0 (both included), more preferably between 0.7 and 0.9 (both included) for a measuring angle at the high-light side {between 10xc2x0 and 25xc2x0 (both included) in terms of open angle from specular reflected light}.
Moreover, it is preferable that the weighted factor ranges between 0.5 and 1.5 (both included), more preferably between 0.8 and 1.2 (both included) at the face side {between 26xc2x0 and 74xc2x0 (both included)}.
Furthermore, it is preferable that the weighted factor ranges between 1.0 and 2.0 (both included), more preferably between 1.2 and 1.7 (both included) at the shade side.